Ha! This is a good idea. Actually I'd count it by the employee pay/cost being soaked up in the meeting instead of an arbitrary rate. So in your case say the average per hour cost to the company for an employee is £50, and there are 10 people in the meeting for an hour, that meeting cost the company £500. It can keep meeting sizes small as a motivator to keep costs low ;)

(I've seen several $20,000-30,000 "meetings" before that accomplished nothing)